1. Field of the Invention
This invention relates to a method of depositing a metal on a surface of a substrate and more particularly, to a method of depositng a metal on a surface of a substrate having an activating metal species distributed in at least a portion thereof.
2. Discussion of the Prior Art
The frequency of use of printed circuit boards has in recent times increased greatly. The advantages of such boards need not be enumerated because they are well known. Various methods for producing metallic patterns on substrates to produce the printed circuit boards are similarly well known. These methods include, alone or in various combinations, positive and negative printing processes, positive and negative silk screening processes, positive and negative etching techniques, electroplating and electroless plating.
Electroless plating has found great favor with many workers in the art. Generally speaking, electroless plating requires a so-called activation or catalization step during which a substrate surface, to be electrolessly plated with a metal has placed thereon a material, usually a metal salt. This metal salt is capable of reducing the plated metal from an electroless bath without the use of an electrical current. Catalization by such a material (called a "catalyst" or an "activator") is referred to as such because the materials used, usually the salts of the precious metals (palladium, platinum, gold, silver, iridium, osmium, ruthenium and rhodium) serve as reduction catalysts in an autocatalytic electroless plating process.
A problem with conventional electroless metal plating of hydrophobic surfaces, such as organic polymer surfaces, is that of adhesion of the metal deposit thereto. Techniques involving etching with acids and/or caustic of the surface prior to activation thereof have been employed with some success. Such etching however often leads to an undesirable amount of surface roughness. An alternative to this etching technique prior to activation is to incorporate and disperse an activating metal species such as palladium metal into the polymer surface either directly or through the use of an ink or resinous covering coat or layer. It has been found however that this incorporation does not lead to a continuous electroless metallization but on the contrary leads to a metallization having voids and more often than not, the incorporation leads to no metallization at all. The resultant voids or lack of metallization (partial or complete) has been hypothesized to be due to the postulated fact that the dispersed palladium species is encapsulated by the polymer and is thus rendered dormant and must then be revitalized or revived to its catalytic state such as by being uncovered or exposed. The revival may be accomplished with mixed success by abrading the polymer surface or etching the polymer surface with mineral acids, e.g., H.sub.2 SO.sub.4, HNO.sub.3. It has been found that this abrading and/or etching not only leads to a surface roughened to an undesirable degree but also to a surface which may not be completely revived, if at all.
An electroless metal deposition process wherein a catalytic species is distributed in at least a portion of the substrate to be metallized and which is subsequently rendered capable at a relatively rapid rate of participating in an electroless metal deposition catalysis without roughening of the surface is therefore desired.